Magnetorheological (MR) fluids are suspensions of finely-divided paramagnetic particles that self-assemble to form chains under the application of an external magnetic field. This aggregation is reversible and, due to Brownian motion, the particles return to their freely dispersed state once the magnetic field is removed. Such chaining of MR fluids in the presence of an external magnetic field has a significant effect on the fluid rheology, including inducing a finite yield stress. Permanently linked chains of these polarizable particles have several interesting potential applications because their suspensions have a dynamic structure in the presence and absence of a magnetic field. S. L. Biswal, A. P. Gast, Physical Review E 2003, 68, 1402-1. They can be used as micromechanical sensors, for shock absorption in automobiles, headrests and ballistic events, and in DNA-separation and other bio-separation processes that use rigid obstacles (e.g., columns of aligned magnetic particles) to impede the convective transport of these species. C. Goubault, P. Jop, M. Fermigier, J. Baudry, E. Bertrand, J. Bibette, Physical Review Letters 2003, 9, 802-1; S. Deshmukh, G. H. McKinley, submitted to Advanced Functional Material; P. S. Doyle, J. Bibette, A. Bancaud , J. L. Viovy, Science 2002, 295, 2237. The rheological properties of these suspensions under different shear and magnetic field conditions may also have unique applications. To date, only flexible chains of superparamagnetic particles, linked permanently by a flexible polymer or other linker, have been reported. S. L. Biswal, A. P. Gast, Physical Review E 2003, 68, 1402-1; C. Goubault, P. Jop, M. Fermigier, J. Baudry, E. Bertrand, J. Bibette, Physical Review Letters 2003, 91, 802-1.
Remarkably, we have invented a technique whereby sol-gel chemistry is used to link permanently polarizable particles that have been aligned within a microchannel by an external magnetic field to yield rigid superparamagnetic chains.